Intensity modulated radiation therapy (IMRT) is an emerging technique in the treatment of tumors. It involves the delivery of many small, concentrated "pencil beams" of radiation that can be varied in intensity. The primary goal of tomotherapy is to deliver a very high dose of radiation to the tumor, while sparing the surrounding tissue. In order to spare the surrounding tissue, this technique causes the high dose of radiation to conform to the shape of the tumor. Tomotherapy delivers a treatment to the tumor in a slice by slice procedure. One thin slice of the tumor is treated at a time before moving on to treat the next slice. The three dimensional shape of the tumor is programmed into a computer which controls the treatment. Because of the high doses of radiation involved and the desirability of having the high dose region essentially the exact size and shape of the tumor, extreme accuracy in positioning the patient is required. The necessary accuracy is maintained by aligning the linear accelerator's isocenter to a precise point within the patient. The isocenter is a single reference point in the x-y-z plane which serves to orient the radiation beams to the proper coordinates of the slices of the tumor.
FIG. 1 shows an example of equipment for performing such a technique. A linear accelerator 10 provides the radiation for the therapy. A multi-leaf collimator 14 concentrates the radiation into the pencil beams which are applied to the tumor. Each pencil beam is approximately one square centimeter in size. The multi-leaf collimator 14 is mounted on the head 13 of the linear accelerator 10 which rotates on a gantry 12 around the patient. The patient is immobilized on a treatment couch 16 which comprises a tabletop 18, a positioning rail 20, and a base 22. The tabletop 18 is able to move independently of the base 22 in the lateral (x-axis) and/or longitudinal (y-axis) directions. The base 22 is moveable in the vertical directions (z-axis). This allows the patient, who is lying on the tabletop, to be positioned to the desired x, y, z location.
It is often advantageous to rotate the treatment couch 16 to a different angle (called a "couch angle") with respect to the linear accelerator 10 in order to treat the tumor from a different direction (i.e., on a different plane). The entire couch assembly may be rotated about the isocenter line of the collimator 15, because it is attached to a rotating disc assembly 24 mounted in the floor. The couch 16 is attached to the edge of the rotational disc 24 such that when the disc 24 is rotated, the couch angle is adjusted accordingly.
Research has shown that there are advantages to delivering IMRT treatments when the isocenter of the linear accelerator is intentionally placed somewhere other than the center of the tumor. For such "non-isocentric" delivery schemes, when the couch angle is changed, basic trigonometry dictates that the treatment couch must make lateral and longitudinal translations to maintain the original isocenter at the same point within the patient. These translations must be done with sub-millimeter accuracy relative to the isocenter. One prior art method performs these translations using a self-supporting positioning device known as the "Crane.TM." which is available from the Nomos Corporation as part of its Peacock System.RTM. radiosurgery device. FIG. 2 shows a side view of the prior art positioning device. FIG. 3 shows the overhead view of the same device. The positioning device 26 comprises a central vertical beam 28 which is supported by three legs 30. The device 26 is moved by retractable wheels 32 which are recessed within the legs 30 when the device is stationary. The central beam 28 supports a lateral positioning arm 34 which extends through a lateral arm housing 35. The location of the lateral positioning arm 34 is adjusted by a lateral movement crank 36. The lateral positioning arm 34 is connected to the longitudinal arm housing 39. A longitudinal positioning arm 38 extends through the longitudidinal housing 39. The location of the longitudinal arm 38 is controlled by a longitudinal movement crank 40. Two attachment arms 42 are mounted on top of the longitudinal arm housing 39. These arms 42 attach to the positioning rail 20 of the tabletop 18 (shown in FIG. 1) with attachment clamps 44 which are tightened with attachment cranks 46.
A device of the type shown in FIGS. 2 and 3 is, by design, very heavy (approximately 300 lbs.). When the couch angle is changed, the positioning device is elevated onto its wheels and dragged or pulled alongside the treatment couch as it rotates to the new angle. When the device is elevated onto the wheels, however, it exerts compressive, tensile, and/or shear forces on the attachment arms. These forces alter the position of the treatment couch relative to the device and, consequently, invalidate any previously set isocenter. Additional large forces also are exerted on the attachment arms when the device is dragged or pushed by the treatment couch while it is moving. These forces alter the position of the couch in relation to the device. Furthermore, the length of the lateral and vertical positioning arms, which is necessitated by the free standing design, makes the device prone to "flexing" of the system, which can induce clinically significant errors. In combination, these forces can, individually and collectively, result in significant errors of as much as 1 cm or more. Therefore, in practice, accurate treatment at multiple couch angles utilizing non-isocentric IMRT delivery schemes are currently not practical.